1988 Mercury 50HP - 3rd cylinder not firing

[JJ-Juniper]

Mercury serial # 0B217717

I'm a new boat owner who's fairly handy, but have just started learning about 2-cycle engines. I'm trying to get this old girl back to proper working condition. I have learned a ton from reading so many posts on this forum. You guys are the best!!

So... here's my problem: Engine starts up easily, but I've found that only cylinders 1 and 2 are firing. I have determined this by doing a cylinder drop test. With engine at a fast idle, I disconnect the wire on the plug for cylinder 3, and the RPMs don't change. If I disconnect plugs 1 or 2 similarly, the RPMs drop noticeably.

I have pulled the cylinder 3 spark plug, and I get what seems to be a good, strong spark to ground when I hold in a bit less than 1" from ground. The tip of plug 3 is way cleaner than 1 and 2, so I suspect it hasn't been firing at all. I had the boat out on the water pulling kids on a tube for an hour when the overheat horn sounded. I shut down right away and brought her in for investigation. With no gas/oil in cylinder 3, I'm hoping that this was the cause of the overheat alarm. Compression in all three cylinders is between 105-108 psi.

I checked my stator, and Ohms are good for red and blue cirucits. DVA readings are both also within spec. When checking blue circuit and increasing RPMs, the voltage climbs to the 280-310 range. I do seem some flickers of readings below 100. They are very brief, and 98+% of the time, the reading is between 280-310. Is this a sign of a bad stator? I have recently replaced the power pack, as my earlier DVA tests (few weeks ago) showed the stator was good, and the pack was not. (I wasn't getting any spark before replacing the power pack.)

I pulled the two carbs out and made sure they look good. All holes that I can see are clear and clean. The float is not getting stuck. If I remove the bottom bolt from the bowl, I can get gas to flow freely when I squeeze the bulb.

I appreciate your thoughts with where I should look next! Thanks in advance!
Jeff

 

[JJ-Juniper]

I'm still struggling with my third valve not firing and am stuck. I appreciate any thoughts you guys have. Here's what else I've done:
- swapped ignition coils, and the problem stays on the lower cylinder
- replaced stator
- swapped plugs 2 and 3, and the problem stayed on the third cylinder
- verified using the CDI mercury troubleshooting manual that all Ohms and DVA voltages are within range at all three coils (200-210V). (I figured out that I wasn't pressing hard enough on the meter leads for the first DVA tests. When I press harder, the DVA readings are steady and in the right range per the CDI guide.)
- disconnected plugs 1 and 2, then was able to start engine and keep it running with just plug 3 connected. I held it at a rough idle for 10 seconds, then was also able to rev up the RPMs to 3500 or so before turning it off
- took it on the water, and cylinder 3 is definitely not firing, as the boat takes way longer to plane than previously. Also, did another cylinder drop test that confirmed no change in RPM when removing plug #3 when running.
- sprayed starting fluid in top and bottom carbs while engine at fast idle, and engine reacted similarly to both short sprays (bogged down, then recovered)
- regarding alarm that went off last weekend, I replaced water pump impeller, and the alarm did not go off yesterday after running at WOT for a while.

Thanks for your thoughts!
Jeff

 

[flyingscott]

Compression test needs to be done.

 

[JJ-Juniper]

Cylinder 1 - 100
Cylinder 2 - 98
Cylinder 3 - 96

I just did a more thorough cylinder drop test.
- Drop #3 - no change in RPM
- Drop #2 - small decrease in RPM
- Drop #1 - large drop in RPM, and engine almost sputters out

I would conclude this tells me I have a fuel issue with my lower carb not supplying enough or too much gas. I tried my best to clean both carbs (especially the lower one). Maybe I need to watch more carb cleaning youtube videos. Thoughts or advice appreciated. Remember, I'm new to this, so can't hurt to run through things that might be obvious to you all!
Thanks!
Jeff

 

[JJ-Juniper]

Hi - does anyone have any suggestions? I'm stumped on this one. I gave the carbs another cleaning, and they looked good. Every hole was clear. I sprayed all the holes with carb clean, ran a wire through each, let it dry, then sprayed it out with compressed air. I didn't replace any gaskets. Is that really necessary if they look intact? I screwed the idle mixture screw (with the spring) all the way in, then backed out 1 3/4 turns. What am I missing?!?

Thanks!

 

[Faztbullet]

1) try a different tester as that compression is way low....2) Lower crank seal allowing ingestion of water which is common problem.

 

[wn6ngp]

I had a similar problem after I rebuilt the fuel pump on my 90hp elpto. I did not pay enough attention to the sequence of assembly of the gaskets and diaphrams. It mostly ran but very rough. The pump seemed to work but apparently did not put out enough pressure to keep the carb bowls evenly filled so 1 cyl did not seem to be firing. I studied the manual (which had lots of different pumps) and got it right finally. next time I'll lay out the parts and take a photo.

 

[JJ-Juniper]

Thanks for the replies!
Faztbullet
1) I re-tested and had readings of 106, 104 and 104. FWIW, I also had a Mercury tech at a Bass Pro Shop test compression about a month ago,and he said compression was good. I didn't ask for the psi then. I had presumed that if all three cylinders have similar psi and one cylinder is firing, that I'm ok. Is that not really the case?
2) How can I test or verify if the lower crank seal is leaking water?
wn6ngp
I have not touched the fuel pump yet. Any thoughts on how to test or verify that? I did already disconnect the fuel hose just after the fuel filter and cranked the engine. There was what I thought was a good stream of fuel being pushed out. I didn't quantify it, but it appeared to be lots more than the engine would use when running.

One other thing that I think is unrelated to this problem, but I'll mention it anyways. I dropped the lower unit and replaced the water pump impeller. I had trouble re-attaching the lower unit (couldn't get the shifting gears and drive gears line up correctly), so that's when I brought it to the Bass Pro tech. He was able to get the lower unit back on. However, when he did, he removed the 2" long rubber tube that connects the outlet on the top of the water pump housing and slides over the end of the 24" or so copper tube that goes to the cylinders. He tried to convince me this piece wasn't necessary and was just there to help align the pipe to the pump housing. I disagreed, thinking that there would be a much bigger gap there that would reduce the amount of water actually making it through the copper tube. Regardless, I took the boat and have been struggling with it since. It does now take at least 60 seconds for water to come out the tattle tale. I can't find this part online to purchase and make him re-install it. Do you think I should bother with this?

I've been doing my testing in my driveway with the engine in a barrel. The barrel how has a lot of oil and gas in it, so I suspect the cylinders that aren't firing are indeed getting fuel/oil - perhaps too much.

Thanks!
Jeff

 

[JJ-Juniper]

I spent a little time playing around this afternoon and found that I can start the engine with any single spark plug connected. The engine sounds similar with any of the 3 plugs connected by itself. Does that tell me anything? Could I have messed up the timing somehow? I know nothing about adjusting timing yet. Should I learn?

 

[JJ-Juniper]

Problem solved - turns out when I received the boat, the wires from the switch box were not going to the correct coils. Got that straightened out, and now it works fine.

 

[aglbrown]

Thanks for this - spent the weekend puzzling about the same on a 96 50p - Turns out the short green-red is not for the top coil it's for the bottom after reading the service manual!